Manufacture of acetic anhydride



NTE A @o :HOWARD W1 MATHESON AND GEORGE E. GRATTN, 0F SEAWINIGAN FALLS,QUEBEC,

CANADA, ASSIGNORS TQ SHAWINEGAN LABORATORIES LIMITED, QF MQNTEEL,

QUEBEC, CANADA.

MANUFACTURE 0F AGETIC ANHYDRIDE,

Application led April 25,

To all whom z'tmay concern:

Be it known that we, HOWARD W. MATHE- soN and GEORGE E. GRATTAN, bothsubjects of the King of Great Britain, and residents of ShawiniganFalls, in the Province of Quebec and Dominion of Canada, have inventedcertain new and useful Improvements in the Manufacture of AceticAnhydride7 of which the 'following is a full, clear, and exactdescription.

'This invention relates to improvements in the manufacture of aceticanhydride, and the object of the invention is to provide for theproduction of this material from inexpensive and easily obtainable rawproducts, in order that manufacture may be carried out at a low cost.

A further object is to provide for the manufacture in a direct, rapidand inexpensive manner. y

`. A still further object is to provide for the manufacture in such away that the apparatus used is comparatively simple and inexpensive.

Acetic anhydride has previously been made for the most part by either oftwov processes which are long and involved, requiring the use ofexpensive equipment. These processes are furthermore not economical.According to one of these processes, acetic anhydride has been made bythe action of phosphorous oxychlorides or similar phosphorus compoundsof acetic acid or acetic acid salts; according to the other process bythe action of sulphur monochloride or similar compounds or anhydroussodium acetate or other suitable salts of acetic acid.

It has also been proposed in patent speciiications to produce aceticanhydride'from acetic acid and acetylene. This method, however, dependson the production of ethylidene diacetate and its decompositionV by theaction of catalysts at a high temperature.

According to the present invention, acetic anhydride is formed in onestep by the action of acetylene gas on commercial glacial acetic acid inthe presence of iinely divided salt of mercury. The acetylene may beused under atmospheric or super-atmospheric pressure. The acid is heatedto a. temperature of from 60 to 200 C. according to the pressure underwhich the reaction Specification of Letters Patent.

v atmospheric.

Patented Aug. S, 11922,

1918. Serial No. 230,807.

takes place, and is kept violently agitated until the absorption ofacetylene is complete. By this process, acetic anhydride is formeddirectly and in a comparatively short time.

In order to facilitate comprehension of the process, a diagrammaticillustration of the apparatus used is annexed.

The following example will serve to explain the process more fully F ivehundred grams of purified acetic acid of 96% to 100% concentration isplaced in a container 1, provided withrelux condensers 2 and 3, theformer of which is preferably a water condenser and the latter a brinecondenser. IThe reaction vessel 1 is provided with cooling coils 4, athermometer 5 and an agitating apparatus 6. The acid is heated to atemperature of about 90 C. and twenty-five grams of mercury oxide addedand dissolved in the acid. Approximately twelve to thirteen grams ofconcentrated sulphuric acid are then slowly added to precipitate themercury as mercuric sulphate. Theseingredients may conveniently beintroduced through the funnel 47 and valve 8. Acetylene gas in excess isthen passed into the reaction kettle through the ipe 9 under a pressureof approximate y forty pounds per square inch above The temperature isallowed tolrise to 130o to 140 C., being regulated by passing waterthrough the cooling coils 4. After a 'period of from one to two hoursthe absorption of acetylene is complete and the mixture in the kettleconsists of acetic anhydride mixed with ethylidene diacetate and a smallamount of acetaldehyde. lDuring the period of absorption, the agitator 6is kept in operation, so as to promote contact of all ortions of theacid. with the acetylene. uring the` process an equal pressure ismaintained in the reaction vessel and condensers'and the major portionof the acetaldehyde produced is collected in a receiver 10 connectedwith the brine condenser 3. Any acetic acid carried over by the excessacetylene is condensed in the condenser 2 and runs back into thereaction through the pipe 11. The water `condenser 2 is kept at anapproximate temperature of 20 to 25 C. From this. condenser, the gaspasses through the pipe 12 to the brine condenser 3, where anyacetaldehyde is condensed and eliminated from the reaction.

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The material in the reaction vessel is then filtered to remove mercurywhich has been reduced in the process and the resultant product ispurified by distillation.

'From the foregoing description, it will be seen that the anhydride isproduced quickly and economically from inexpensive and readilyobtainable materials and by the use of a sim le and inexpensiveapparatus. The anhy ride is furthermore produced directly, so that theproduction is much more economical than is possible by the indirectmethods previously referred to. Unabsorbed acetylene which escapes fromthe brine condenser may be returned into the reaction vessel 1 by meansof a pump or other suitable apparatus.

It has been stated in the foregoing example that the pressures in thereaction vessel and condensers are equal. This condition is preferred asthe process may be more easily carried out and the apparatus is moresimple, but it will be obvious that the process may be carried out underdiierent pres' sures.

While the example describes mercurio sulphate as the catalyst, it willbe understood that the invention is not limited to this par-4 ticularsalt but includes any suitable mercury salt introduced as such orproduced in the reaction liquor.

The best results are obtained by using the specific temperatures andconcentrations given in the example, but it has been found by experimentthat other temperatures and various degrees of 'concentration may beused and the desired result obtained.

lt has been stated in the foregoing example that the pressures in thereaction vessel and condensers are equal. This condition is preferred asthe process may be more easily carried out and the apparatus is moresimple, but it will be obvious that the process may be carried out underdiferent pressures.

Having thus described our invention, what we claim is;-

1. A process for the manufacture of-acetic anhydride, which comprisespassing acetylene through acetic acid of 96% to 100% concentration at atemperature of from 'centigrade to 200 centigrede in the presence of 'afinely divided salt of mercury.A

2. A process according to claim- 1, in which the acid is maintained inviolent agitationduring passage of the acetylene therethrough.

3. A process for the manufacture of acetic anhydride, which comprisespassing acetylene gas in excess of the combinable amount through aceticacid of 96% to 1.00% concentrataion at a temperature between 60 and 200centigrade, and separating entrained acetic acid from the'excessacetylene by condensation, and returning the acid into the reaction.

4. lln combination with a process according to claim 3, the additionalstep of separating acetaldehyde formed in the reaction from excessacetylene by a condensation separate from that employed to separate theentrained acid.

5. A process for the manufacture of acetic anhydride which comprisessuccessively adding approximately two parts. of mercury oxide andapproximately one part of con.

centrated sulphuric acid to forty parts of acetic acid of 96% to 100%concentration, passing acetylene gas through the mixture, andmaintaining a temperature between 60 and 200 'centigrade 6. A processfor the manufacture of acetic anhydride, which comprises heating aceticacid of 96% to 100% concentration to a temperature of approximately 90centi grade, successively adding mercury oxide and sulphuric acid,passing acetylene gas through the mixture, and maintaining a temperaturebetween 60 and 200o centigrade for the period of the reaction.

7. In combination with Ia process according to claim v6, separatingentrained acetic acid and acetaldehyde f ormed in the reaction bysuccessive condensing operations, the first condensation being carriedout at a temperature above the condensation point of acetal dehyde andbelow the condensation point of acetic acid, and returning the aceticacid recovered to the reaction.

8. A process according to claim 1, in

-which the reaction is carried out under super-atmospheric pressure.

9. A process according to claim 1, carried out at a pressure ofapproximately 40 pounds per square inch super-atmospheric.

ln witness whereof, we have hereunto set our` hands.

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